72 



J. B. CHAPPELL 



shown in Fig. i [i]. Most experiments were performed with rat liver 

 mitochondria isolated in 0-25 M-sucrose containing 5 mM-2-amino- 

 hydroxymethylpropane-i :3-diol-hydrochloride (tris) buffer, pH 7-4, but 

 some results obtained with kidney mitochondria prepared in the same 

 medium are presented. 



Isocitrate oxidation 



When the oxidation of isocitrate by liver mitochondria was followed 

 in a medium containing 80 mM KCl, 6 mM MgClg, 15 mM phosphate and 

 ID mM substrate and respiration was stimulated by addition of small 

 quantities of adenosine-diphosphate (ADP) an unusual pattern resulted 



12 3 4 5 

 Time (mm) 



Fig. 2. Stimulation by ADP of glutamate and isocitrate oxidation in a medium 

 containing 15 mM-phosphate. The numbers juxtaposed to the curves in this and 

 subsequent figures represent Q,,., (n) values (/d. O.i/mg. N/hr.). 



(Fig. 2). When L-glutamate, a-ketoglutarate, /3-hydroxybutyrate, proline 

 or succinate served as substrates the State 3 rate [2] was linear until nearly 

 all the ADP had been converted into adenosine triphosphate (ATP), 

 when the rate characteristic of State 4 ensued. With Lg( + )-isocitrate as 

 substrate after a short period in State 3 the rate of oxidation declined and 

 this lower rate persisted until the added ADP was exhausted. A subsequent 

 addition of ADP after a short period in State 4 led to a further rapid rate 

 of oxidation followed by a slower rate. However, the longer the period of 



